Chirped surface acoustic wave (SAW) correlator/expander

ABSTRACT

A surface acoustic wave (SAW) expander based transmitter and correlator based receiver comprises SAW devices that perform expander or correlator functions based on the types of signals inputted to the SAW devices. The SAW devices incorporate chirp with adaptive interference and programmable coding capabilities. The SAW devices and method of operating the devices allow the implementation of very low power radios that overcome problems with temperature drift, lithography constraints and interference and jamming suffered by prior art implementations.

1. RELATED APPLICATION DATA

This application claims priority from U.S. Provisional Application No.60/558,173 filed on Mar. 26, 2004.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to electronic circuits and, moreparticularly, to circuits for wireless communication.

2. Description of the Related Art

Conventional radios have been extensively implemented usingsuperhetrodyne and direct conversion architectures constructed fromcircuit blocks such as mixers, amplifiers, and RF filters. There aremany chipsets on the market using various process technologies, but theyare usually active circuits requiring a power source. One alternateapproach is to construct a radio using a passive surface acoustic wave(SAW) expander and correlator.

FIGS. 1 and 2 show a representative transmitter 10 and receiver 20,respectively, using the SAW approach. The main disadvantage of SAWcorrelator and expander implementations in the prior art is that theyare implemented at an intermediate frequency (IF) to allow a localoscillator (LO) (12, 22) to compensate for inherent temperature driftand tolerances of the SAW device (11, 21) as a closed loop system.Although the SAW device (11, 21) itself is passive in thisimplementation, other active circuits are still required at the frontend and for the LO and up/down converters, as shown. Without suchcompensation, the correlation performance and sidelobe levels becomeunacceptable over a practical temperature range, making the radioperformance unacceptable. Historically, correlators/expanders have notbeen implemented directly at the radio frequency (RF) front end becauseof the temperature drift problem described above, but also because ofproblems with device to device manufacturing variance and difficultieswith small feature size lithography required for operation above about800 MHz.

Thus, it would be desirable to have a correlator/expander implemented atthe RF front end if the issues of temperature variation and tightlithographic technology could be resolved.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a communications system fortransmitting and receiving signals comprises a baseband controller forcoding a transmit signal and decoding a receive signal, a switch networkcoupled to the baseband controller, and surface acoustic wave (SAW)devices coupled to the switching network. On the transmitter side, areference oscillator provides a plurality of reference signal pulses tothe switch network. The SAW devices perform expander or correlatorfunctions based on types of signals inputted.

In another embodiment of the invention, a surface acoustic wave (SAW)expander based transmitter comprises a baseband controller for coding aninput signal and providing an output signal to a switching network, areference oscillator for providing a plurality of reference signalpulses to the switching network, first and second chirp expanderscoupled to the switching network for forming first and second chirpedsignals, and a combiner circuit for combining the first and secondchirped signals into a transmit signal.

In another embodiment of the invention, a surface acoustic wave (SAW)correlator based receiver comprises: a circuit for receiving a signal, asplitter for splitting the received signal into first and secondsignals, first and second chirp correlators for compressing the firstand second signals and forming first and second correlated pulses, and abaseband controller for decoding the correlated first and second pulsesand forming a receive signal.

In yet another embodiment of the invention, a method of transmitting andreceiving signals in a communications system comprises providing controlsignals to a switching network, and operating surface acoustic wave(SAW) devices coupled to the switching network. At a transmitter side,the method comprises generating a plurality of reference signal pulsesand providing the pulses to the switching network. The SAW devices areoperated at a radio frequency (RF) front end of the communicationssystem. Active compensation schemes are not used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conventional surface acoustic wave (SAW) expander basedtransmitter.

FIG. 2 is a conventional SAW correlator based receiver.

FIG. 3 is an improved SAW expander based transmitter.

FIG. 4 is an improved SAW correlator based receiver.

FIG. 5 is a block diagram of chirp SAW correlator/expander functionsnecessary for a radio implementation.

FIG. 6 is a simplified schematic of SAW expander and correlator showinginput and output signal transforms.

FIG. 7 is a diagram of slanted IDT.

FIG. 8 is a SAW based radio schematic transmitting up and down chirps.

FIG. 9 is a simplified SAW based radio schematic transmittingunidirectional chirps.

FIG. 10 is a conceptual layout of SAW correlator/expander (fundamentaldimensions).

DETAILED DESCRIPTION OF THE EMBODIMENTS

An embodiment of an improved surface acoustic wave (SAW) expander basedtransmitter 30 is shown in FIG. 3. The expander based transmitter 30comprises a baseband section 31, an oscillator 32, two switches 33, twochirp expanders 34, 35 (of opposite chirp direction) and a combiner 36.Data is suitably coded in the baseband section 31 and its outputcontrols the switches 33. Data is imparted to the transmitter 30 bycreating a train of pulses from the reference oscillator 32. The pulsesare fed to the chirp expanders 34, 35 to create an up or down rampingfrequency pulse. The expanders 34, 35 are fabricated on a surfaceacoustic wave (SAW) substrate with novel interdigital transducers(IDTs). One IDT is used to implement programmable coding formultiple-access and the other is used to implement the chirp. Afterpassing through the expanders 34, 35, the chirped signals are thencombined in combiner 36 to an antenna 37 for transmission.

Similarly, an improved SAW correlator-based receiver 40 is shown in FIG.4. The receiver 40 comprises a splitter 46, two chirp correlators 44, 45(of opposite chirp direction), adaptive gating blocks 43, demodulationcircuits 42, and a baseband section 41. Following the antenna 47, thereceived signal is split into two signals. The chirp correlators 44, 45will then compress the signals according to the characteristics of thechirp design. Signals matching the direction and characteristics of thechirp correlators 44, 45 will result in a correlated output. All othersignals will be uncorrelated. The chirp correlators 44, 45 areimplemented using a SAW device with IDTs designed for coding and chirpfunctions, similar to the SAW expanders 34, 35. The chirped transducerin the correlator is broken into several sub-bands to allow adaptiveinterference mitigation. This is simply implemented by breaking the IDTbus bar into sections. The sub-bands (sections) can be combined ordeleted to mitigate interference or jamming. This function is depictedby the adaptive gating block 43 in FIG. 4. Following this block, thesignals can be demodulated in demodulator 42 using a simple thresholddetector. The detected output can then be decoded by the basebandsection 41.

Together, FIGS. 3 and 4 show an improved system implementation of achirp SAW correlator and expander in a dual channel receiver andtransmitter. In the improved system implementation, the SAW devices arelocated directly at the RF front end instead of at an intermediatefrequency (IF). In addition, no local oscillator (LO) or up/downconverter is required. Furthermore, the SAW expander based receiver 40requires no reference oscillator at all.

FIG. 5 shows a simplified block diagram of the chirped SAWcorrelator/expander functions necessary to implement the transmitter 30and the receiver 40 in FIGS. 3 and 4. The transmitter 30 comprises twoexpanders 34, 35, one with up-chirp (increasing frequency) and one withdown-chirp (decreasing frequency), representing a logical 1 or 0,respectively. In an alternate embodiment, the up-chirp may representlogic ‘0’ and the down-chirp may represent logic ‘1’. Similarly, thereceiver 40 comprises two correlators 44, 45 performing the correlationfunction. The receiver SAW devices are paired with those in thetransmitter, such that the receiver has the inverse time response of thetransmitter.

Advantages of the improved architecture include the ability of chirp tocompensate for temperature drift and process variances by using chirp tosweep over a band of frequencies sufficient to overcome the expectedtemperature drift and manufacturing tolerances of the SAW device. Thiseliminates the need for active compensation schemes such as downconversion and tracking LOs used in conventional non-dispersivearchitectures. Consequently, the correlator/expander can be implementeddirectly at the RF front end, resulting in very low power consumption.In addition, chirp improves the impedance match of the devices, therebyimproving insertion loss.

Another unique advantage of the improved architecture includes theability to combine phase coding and chirp IDTs. A SAWcorrelator/expander comprises an input and output IDT. The combinationof one coded transducer and one chirped transducer to achieve a codedchirped signal in a single device has the advantage of size reductionand lower insertion loss compared with two devices implementing thesefunctions separately. The finger length of the proposed IDTs can beadjusted using a suitable apodization to modify the envelopecharacteristic for maximum sidelobe suppression. Apodization is awell-known window function applied in communication theory. In the caseof a SAW device, the window function can be implemented by adjusting thefinger length across the IDT according to a suitable window(apodization) function.

FIG. 6 shows two SAW devices 50, 60 and depicts the physicalimplementation of the correlators 44, 45 of receiver 40 and expanders34, 35 of transmitter 30. The input IDTs 51, 61 are phase coded with Nnumber of symbols to allow multiple access coding (CDMA). The outputIDTs 52, 62 are chirped in symmetrically opposite directions and brokeninto M sub-bands, which can be summed or deleted forinterference/jamming mitigation. The two SAW devices 50, 60 depicted inFIG. 6 are capable of performing all four of the correlator and expanderfunctions outlined in FIG. 5. The function of each SAW device depends onthe type of signal that is presented to its input terminals (on theleft). If a pulse is presented to the input terminals of either SAWdevice 50, 60, the device will perform as an expander and a chirpedsignal will appear at the output terminals. In this case, the SAWdevices 50 and 60 will have up and down chirps, respectively. Similarly,if upchirp and downchirp signals are presented to the inputs of the SAWdevices 50 and 60, respectively, the outputs will be correlated pulses.That is, the device performs the function of a correlator. Any othersignal combinations will result in uncorrelated outputs.

FIG. 7 shows a further unique feature of the improved architecture,which uses slanted fingers 71 in the phase coded IDT 70 to improvebandwidth. The 4 dB bandwidth of an IDT is approximately 1/Np (where Npis the number of finger pairs). Therefore, for phase coded IDTs withconstant finger spacing, the longer the code, the narrower the bandwidthof the IDT. This dependency can be overcome by using slanted fingers.Slanted IDTs have been demonstrated with maximum slant angles of up to 7degrees for standard (non-coded) IDTs, but have not been applied tocoded IDTs. The coding of the IDT 70 is achieved by the periodicity offinger connections to the bus bars 72, 73. The upper 72 and lower 73 busbars of each symbol (group of fingers) are exactly out of phase, so thatdriving the bus bars with the appropriate polarity will create a phaseinversion in the signal representing coded symbols.

FIG. 8 shows a preferred embodiment of a complete transmitter/receiverradio architecture 80. In particular, the architecture shows the uniqueuse of the SAW correlator/expander 81 for both transmit and receivefunctions, depending on the signal that is switched to the input of thedevice 80. Note that F and F in FIG. 8 represent differential signalinputs. The switch network 82 includes the switching necessary toimplement the programmable symbol coding and adaptive interferencemitigations functions by controlling the signal connections to the IDTbus bars.

FIG. 9 shows an alternate simplified embodiment of the improvedarchitecture. The radio architecture of FIG. 9 employs unidirectionalchirp only, to simplify the switching networks. Either up or down chirpmay be implemented. The architecture uses each SAW device for eithertransmit or receive, but not both (unlike FIG. 8). Thus, the overalldata throughput for FIG. 8 will be halved, all other things being equal,and an alternate signaling methodology would be required since theimplementation of FIG. 9 has only downchirps (or upchirps). Any standardsignal protocol (for example, on-off keying (OOK)) may be used tosignify a ‘1’ or ‘0’. FIG. 9 may implement bi-direction chirps byduplicating the entire architecture with opposite chirp. A commonbaseband controller 95 may be used.

A further enhancement of the improved architecture is operation of theSAW devices at a harmonic frequency. Harmonic operation has theadvantage of reducing lithographic demands, especially for highoperating frequencies (for example, those greater than 800 MHz). In apreferred embodiment, the improved architecture operates at the thirdharmonic, but the use of other harmonics may also be practical. Harmonicoperation also improves the impedance match of the devices (in additionto that from chirp), thereby improving insertion loss. Harmonicoperation for SAW correlators/expanders in radio transmitter andreceiver applications reduces lithographic demands and improvesinsertion loss and is unique.

FIG. 10 is a conceptual layout of a preferred embodiment of the SAWexpander/correlator 100 along with the preferred dimensions. FIG. 10provides a more detailed representation of the SAW device 50 in FIG. 6.The layout and dimensions (and related discussion) of the embodiment arereferenced to a fundamental center frequency of 2441.75 MHz. This can beeasily transformed into a third harmonic design as the final step bysimply omitting every second and third split electrode pair and triplingthe width/spacing of the remaining fingers.

The design of the SAW device 60 in FIG. 6 is very similar to theexpander/correlator 100 shown in FIG. 10, except with opposite chirp.All other aspects of the device are identical and, therefore, notrepeated here. The substrate is assumed to be X112° rotY—LiTaO₃, withv=3276.6 m/s, though other piezoelectric substrates may be used inalternate embodiments.

Referring back to FIG. 10, the input IDT 110 is split into 32 symbols toallow phase coding. In the embodiment shown in FIG. 10, a 32 symbol codeis used. In another embodiment, N symbol code may be used, where N is aninteger. The bus bar 112 is split between symbols 120 to permit thepolarity to be arbitrarily set (programmed) with an external switchingnetwork. Each symbol 120 is chosen to be 24) long. Ideally, a smallersymbol length would be preferred since the design is severely padlimited. However, 24λ is 32.2 microns long, which is approximately thesmallest practical staggered pad pitch supported by modern wire bondingequipment. The symbol size must be exactly divisible by 3λ to allowscaling to the third harmonic as the final step, if desired.

The input IDT 110 uses a split finger design (4 fingers per λ) tominimize reflections. The line/spacing dimension (Wc) is λ/8 or 0.1677microns at the center frequency (2441.75 MHz), which is beyond currenthigh volume manufacturing limits. As mentioned above, this may bemitigated in the final layout by omitting every second and third tap andtripling the width/spacing of the remaining taps to produce a thirdharmonic design.

The center frequency wavelength is given by λ=v/f=1.342 microns, wherev=3276.6 m/s for X112° rotY—LiTaO₃ and f=2441.75 MHz. Therefore, thenominal length of the input IDT 110=32 symbols×24λ long=1.0306 mm.

Unfortunately, a regularly spaced input IDT cannot be used. The largenumber of taps limits the bandwidth of the IDT to approximately 1/Np(where Np is the number of finger pairs; each finger is implemented as asplit finger in this case). This is the same as the total number ofwavelengths and gives an approximate bandwidth of 1/768 or about 0.13%,much too narrow to pass 60 MHz (or 2.5%) desired for this embodiment.

A method to increase the inherent bandwidth of an IDT is to use slantedfingers, as shown in FIG. 7. The fingers 71 are slanted to a commonfocal point, so that there is a constant minimum width (Wmin) at the topof the IDT fingers and a constant maximum width (Wmax) at the bottomend. These two widths are then set to the upper and lower frequencies,respectively. In this way, the fingers 71 are resonant at some pointalong their length across the desired bandwidth.

The width/spacing of the slanted IDT is given by the equations:Wmax=λ(min)/8=v/8f(max) andWmin=λ(max)/8=v/8f(min),such that the minimum width/spacing Wmin=0.1657 microns and the maximumwidth/spacing Wmax=0.1698 microns at the fundamental frequency in FIG.7.

The slanted IDT 70 in FIG. 7 trades the effective aperture for bandwidthsince the IDT is resonant only over a small part of the IDT finger 71.This effectively increases insertion loss. However, the reducedcapacitive loading should improve the impedance match of the device tosomewhat offset this.

Referring back to FIG. 10, the output IDTs 130 of the two SAW devicesused for this system are a mirror image of each other, one with upchirpand one with downchirp. The finger spacing, Wn, on the output increasesor decreases linearly, depending on the direction of chirp. The bus bars132 on the output transducers 130 for this embodiment are broken into 16sub-bands (equivalent to 3.75 MHz) to facilitate adaptive interferencecancellation. The sub-bands 140 can be summed or deleted in response tothe jamming environment. As noted earlier, the four correlator/expanderfunctions can be accomplished using just two SAW devices because the SAWcorrelators used for the receiver can also be used to transmit (expand)the opposite channel in the transmitter. However, in the case oftransmission, the sub-bands of the chirped IDT are all combined into onein the embodiments of FIGS. 3, 8, and 9. In an alternate embodiment, thesub-bands may be summed or deleted in order to avoid transmitting insub-bands known to include interference or jammers.

The length of the dispersive IDT sets the length of the frequency ramp(chirp length), whereas the delay is set by the distance between thecenters of the input and output IDTs. For convenience, we choose thelength of the input and output IDTs to be the same (i.e., 768λ). Thisnumber of wavelengths is convenient for the output IDT 130, since thenumber is readily divisible into 16 sub-bands and also scales with aninteger relationship to the third harmonic. It can be shown that thetotal length of the chirp IDT is the same as for a non-chirp input IDT(1.0306 mm), corresponding to a chirp time of 0.3145 microseconds.

The data rate is given by the reciprocal of the delay. For a 3 Mb/s datarate, a delay of ⅓ microsecond is required, which approximatelycorresponds to a distance:(v*t)=1.0922 mm,while the total delay path length is given by:Total delay path length=½(Input IDT length+Output IDT length)+IDT Gaplength.Therefore, for a ⅓ microsecond delay and equal IDT lengths (1.0306 mm),the IDT gap is:1.0922−½(1.0306+1.0306)=0.0616 mm.

The selection of apodization and IDT aperture size is adjustabledepending on the desired specifications. In this embodiment, thecombined IDT response has a response equivalent to a Hamming functionwith 42 dB minimum sidelobe suppression.

Due to the high operating frequency, care must be taken with thephysical layout to minimize RF losses, parasitics, and undesiredcoupling. The output traces and bond pads should be kept as short aspossible to minimize the series inductance. Mutual coupling betweenadjacent I/Os should be minimized. In general, good RF practice shouldbe exercised in the overall layout methodology to minimize the parasiticeffects of the I/O. Special attention must be paid to parasitic couplingbetween bond pads and to traces passing between bond pads.

It should be appreciated that reference throughout this specification to“one embodiment” or “an embodiment” means that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present invention.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the invention.

Similarly, it should be appreciated that in the foregoing description ofexemplary embodiments of the invention, various features of theinvention are sometimes grouped together in a single embodiment, figure,or description thereof for the purpose of streamlining the disclosureaiding in the understanding of one or more of the various inventiveaspects. This method of disclosure, however, is not to be interpreted asreflecting an intention that the claimed invention requires morefeatures than are expressly recited in each claim. Rather, as thefollowing claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the claimsfollowing the detailed description are hereby expressly incorporatedinto this detailed description, with each claim standing on its own as aseparate embodiment of this invention.

1. A communications system, comprising: a surface acoustic wave (SAW) expander based transmitter, wherein the transmitter comprises: a baseband controller for coding an input signal and providing an output signal to a switching network; a reference oscillator for providing a plurality of reference signal pulses to the switching network; first and second chirp expanders coupled to the switching network to receive input pulses for forming first and second chirped signals; and a combiner circuit for combining the first and second chirped signals into a transmit signal; wherein at least one of the chirp expanders includes: an input interdigital transducer (IDT) having a plurality of input symbol segments to enable phase coding of the input signal, wherein the input IDT comprises fingers having top and bottom ends, wherein the fingers are slanted with the top ends directed to a common focal point, the top ends have a constant minimum width, and the bottom ends have a constant maximum width; and wherein individual input symbol segments comprise a first group of said fingers extending from an upper bus bar towards a lower bus bar, and a second group of said fingers extending from the lower bus bar towards the upper bus bar, fingers in the first group of fingers interdigitated with fingers in the second group of fingers, fingers in the first group of fingers having top ends connected to said upper bus bar, and fingers in the second group of fingers having bottom ends connected to said lower bus bar.
 2. The system of claim 1 wherein at least one of the chirp expanders further includes: an output IDT for converting the surface acoustic wave into an electrical signal, the output IDT broken into sub-bands.
 3. The system of claim 1, further comprising: a surface acoustic wave (SAW) correlator based receiver, wherein the receiver comprises: a circuit for receiving a signal; a splitter for splitting the received signal into first and second signals; first and second chirp correlators for compressing the first and second signals and forming first and second correlated pulses; and a baseband controller for decoding the correlated first and second pulses and forming a receive signal; and further wherein each of the chirp correlators includes an output IDT broken into sub-bands.
 4. The system of claim 3, wherein the first and second chirp correlators are chirped in symmetrically opposite directions.
 5. The system of claim 3, wherein the first and second chirp correlators are surface acoustic wave (SAW) devices, each device comprising: an input interdigital transducer (IDT) for generating a surface acoustic wave, the input IDT phase coded with N number of symbols, where N is an interger; and an output IDT for converting the surface acoustic wave into an electrical signal, the output IDT broken into sub-bands.
 6. The system of claim 1 wherein: the input interdigital transducer (IDT) comprises a plurality of input symbol segments formed by splitting upper and lower bus bars so as to form an upper bus bar segment and a lower bus bar segment for each symbol position; the upper bus bar segment and the lower bus bar segment for each symbol position comprising a respective group of fingers and a respective electrical connection to the group of fingers; and the switching network is arranged for selectively coupling an input symbol to anyone or more of the upper and lower bus bar segments of the input IDT so as to apply a desired access coding.
 7. The system of claim 6 wherein the access coding is achieved by a selected periodicity of the finger connections to the bus bars.
 8. The system of claim 6 wherein the upper and lower bus bars of each symbol group are out of phase so that driving the bus bars with a selected polarity creates a phase inversion in the signal representing the access coded symbols.
 9. A method comprising: providing control signals to a switching network; at a transmitter side, generating a plurality of reference signal pulses and providing the pulses to the switching network; operating first and second passive surface acoustic wave (SAW) devices coupled to the switching network, the first SAW device including an input interdigital transducer (IDT) having a plurality of input symbol segments to enable phase coding of an input signal, wherein the input symbol segments of the input IDT include fingers slanted to a common focal point, wherein top ends of the fingers have a constant minimum width and bottom ends of the fingers have a constant maximum width; and wherein said operating the first SAW device includes selecting a desired phase coding, and operating the switch network so as to couple the input signal to selected input symbol segments of the input IDT in accordance with the selected phase coding; and wherein the input IDT is arranged to enable phase coding of the input signal by providing a respective group of interdigitated fingers for each input symbol, wherein each symbol group of fingers comprises a corresponding upper bus bar and a corresponding lower bus bar, the upper bus bar extending to form a first series of slanted finger connections, and the lower bus bar extending to form a second series of slanted finger connections interdigitated among the first series of finger connections, and further wherein the upper and lower bus bars are positioned out of phase.
 10. The method of claim 9, wherein the first and second SAW devices are operated at a radio frequency (RF) front end of a wireless radio frequency (RF) communications system.
 11. The method of claim 9, wherein the first and second SAW devices are operated at an operating frequency which is a harmonic of a desired frequency.
 12. The method of claim 11, wherein the operating frequency is a third harmonic of the desired frequency.
 13. The method of claim 9, wherein operating the first and second surface acoustic wave (SAW) devices comprises: operating a phase coded interdigital transducer (IDT); and operating a chirp IDT.
 14. The method of claim 9, wherein active compensation schemes are not used.
 15. A transceiver comprising: a first surface acoustic wave (SAW) device comprising a first input interdigital transducer (IDT) and a first output IDT, wherein the first input IDT is divided into a first plurality of symbol groups of fingers for multiple access coding; a second SAW device comprising a second input IDT and a second output IDT, wherein the second input IDT is divided into a second plurality of symbol groups of fingers for multiple access coding; wherein the first and second input IDTs are phase coded with N symbols, where N comprises an integer, and the first and second output IDTs are chirped in symmetrically opposite directions and broken into an integer number M sub-bands; wherein the first SAW device is arranged to respond to an input data pulse as an expander and also to respond to a received signal as a correlator depending on a signal switched to the input; and wherein the second SAW device is arranged to respond to an input data pulse as an expander and also to respond to a received signal as a correlator; and a switching network operatively coupled to both the first and second SAW devices for controllably directing signals between the SAW devices and a baseband controller of the transceiver, wherein the switching network is arranged for selectively connecting an input signal to one or more of the symbol groups of fingers for expanding the signal in accordance with a desired access code.
 16. The transceiver of claim 15, wherein the first input IDT and the second input IDT each comprise IDT fingers having top and bottom ends, the IDT fingers are slanted to a common focal point, the top ends have a constant minimum width, and the bottom ends have a constant maximum width.
 17. The transceiver of claim 15 wherein the switching network is arranged for selectively connecting the received signal to one or more of the output IDTs for correlation to a predetermined access code. 